Monitoring of Crassostrea gigas performances, from two year-classes, during 12 years, in various French shellfish farming areas have revealed great inter-annual and inter-sites differences in reproduction, growth, mortality or quality of oysters (meat content, Polydora infestation...). The presentation attempts to highlight how a comparative analysis of existing temporal or spatial data on molluscs, water quality, geography or meteorogoly may help to better understand and manage shellfish ecosystems. The range of hydrobiological conditions (food availability, turbidity, temperature...) recorded at national, pluriannual scale, in relation to shellfish performances, enlarges the base of such relationship. However, undirect causative factors, either natural (geography, meteorology) or human (cultured stocks, densities), may present advantages in terms of accessibility, determinism (closer to source) and management : - the observed «site effect » on growth and mortality is not clearly linked to geographical traits (morphology, oceanic exposure, island protection...). More significantly, the vicinity of fluvial nutritive fluxes (Gironde, Loire...) determines nearby shellfish production but also a higher variability (year-site interaction): oyster growth fluctuates more through successive years in south-Brittany than in north-Brittany, probably in closer response to climatic and fluvial variations. Fluvial inputs in excess during rainy years seem to increase mortality risk (Over-mortality of one-year oysters in Brittany in 1995 and 2001; of two-years old oysters in a Normandy bay). - meteorological conditions influence oyster production at different stages : rainy springs and hot summers for instance favor oysters reproduction (at Arcachon), but impair one year spat survival, the same year. - According to sites, growth limitation and increased mortalities may occur from stock excess, at different densities and different scales from local (Baie du Mont Saint Michel) to global (Marennes-Oléron). However, physical limitations (farming limited to banks in rias...) or social ones (shellfish leases regulated in Thau laguna...) may have prevented overexploitation.